Chemistry • Year 12 • Module 7 • Lesson 12
Reactions of Alcohols
Lock in the vocabulary, reagents, conditions, and products for dehydration, substitution with HX, and oxidation of alcohols before attempting application questions.
1. Term–definition match
The ten definitions below are shuffled. In the right-hand column write the matching term from this list: dehydration, elimination, nucleophilic substitution, haloalkane, aldehyde, ketone, carboxylic acid, distillation, reflux, acidified dichromate. 10 marks
| # | Definition (shuffled) | Matching term |
|---|---|---|
| 1.1 | The reaction type in which a small molecule (water) is removed from a larger molecule to form a double bond. | |
| 1.2 | The reaction in which the –OH group of an alcohol is replaced by a halogen when treated with HX, producing water as a by-product. | |
| 1.3 | An organic compound containing the –CHO functional group, produced by gentle oxidation of a primary alcohol. | |
| 1.4 | An organic compound containing the C=O group flanked by two carbon groups; produced by oxidation of a secondary alcohol. | |
| 1.5 | An organic compound containing the –COOH group; produced by excess oxidation of a primary alcohol under reflux. | |
| 1.6 | An organic compound in which one or more hydrogen atoms have been replaced by a halogen atom. | |
| 1.7 | The reaction category that covers both dehydration and the formation of HX from a hydrogen halide; a bond breaks while two species separate. | |
| 1.8 | Laboratory technique in which volatile products are continuously removed from the reaction flask as they form, preventing further reaction; used to collect aldehydes from primary alcohols. | |
| 1.9 | Laboratory technique in which vapour is condensed and returned to the flask, keeping all components in contact; used to drive oxidation of primary alcohols all the way to carboxylic acids. | |
| 1.10 | The reagent K2Cr2O7/H2SO4 used to oxidise alcohols; changes from orange to green when oxidation occurs. |
2. True or false — with correction
For each statement, circle T or F. If the statement is false, rewrite it correctly on the line below. 10 marks (1 T/F + 1 correction where needed)
2.1 Dehydration of an alcohol uses dilute sulfuric acid at room temperature. T / F
2.2 The equation CH3CH2OH + HBr → CH3CH2Br + H2O correctly represents a substitution reaction. T / F
2.3 Tertiary alcohols produce a ketone when treated with K2Cr2O7/H2SO4. T / F
2.4 When K2Cr2O7/H+ solution stays orange after being added to an alcohol, this indicates oxidation has occurred. T / F
2.5 Using reflux rather than distillation when oxidising a primary alcohol drives the reaction to give a carboxylic acid rather than an aldehyde. T / F
3. Fill in the blanks
Use the word bank to complete the paragraph. Each word is used once. 8 marks
Word bank: alkene • concentrated • dehydration • distillation • green • haloalkane • orange • reflux
Alcohols undergo three main reactions studied in this lesson. In (3.1) _______________________, the –OH group and an adjacent –H are removed together as water, forming an (3.2) _______________________ with a C=C double bond. This reaction requires (3.3) _______________________ sulfuric acid as the catalyst at approximately 170°C. When an alcohol is treated with a hydrogen halide such as HBr, the –OH is replaced by the halogen in a substitution reaction, producing a (3.4) _______________________ and water. The oxidation of a primary alcohol can be directed to give an aldehyde by using (3.5) _______________________ apparatus, which removes the volatile aldehyde product immediately. Using (3.6) _______________________ apparatus instead keeps the aldehyde in contact with excess oxidant, converting it further to a carboxylic acid. In both cases, the acidified dichromate reagent K2Cr2O7/H+ changes from (3.7) _______________________ to (3.8) _______________________ as Cr2O72− is reduced to Cr3+.
4. Function recall
Answer each in 1–2 sentences using precise terms from Lesson 12. 8 marks, 2 each
4.1 Why does dehydration of an alcohol require concentrated acid rather than dilute acid?
4.2 Why is water always produced as a by-product when an alcohol undergoes substitution with HX?
4.3 Why do tertiary alcohols not react with K2Cr2O7/H+?
4.4 State the reactivity order of the three hydrogen halides (HI, HBr, HCl) in substitution reactions with alcohols, and give the reason.
5. Connect the reaction map
Draw labelled arrows between the six terms below to show how an alcohol can be converted into each product. Each arrow must carry a reaction name and the key reagent or condition (e.g. “dehydration • conc. H2SO4, 170°C”). Aim for at least 5 labelled arrows. 5 marks
Supplied terms: ALCOHOL • ALKENE • HALOALKANE • ALDEHYDE • KETONE • CARBOXYLIC ACID
Q1 — Term–definition matches
1.1 dehydration • 1.2 nucleophilic substitution • 1.3 aldehyde • 1.4 ketone • 1.5 carboxylic acid • 1.6 haloalkane • 1.7 elimination • 1.8 distillation • 1.9 reflux • 1.10 acidified dichromate
Q2 — True / false
2.1 False. Dehydration requires concentrated sulfuric acid (or concentrated phosphoric acid) at approximately 170°C; dilute acid at room temperature would instead drive hydration (addition of water across C=C).
2.2 True. The equation is balanced and correctly shows substitution of –OH by –Br with water as a co-product.
2.3 False. Tertiary alcohols do not react with K2Cr2O7/H2SO4; the solution stays orange. No H atom is available on the C–OH carbon for oxidation to proceed.
2.4 False. Orange K2Cr2O7/H+ that stays orange indicates no oxidation has occurred (consistent with a tertiary alcohol or no oxidisable alcohol being present).
2.5 True.
Q3 — Cloze
3.1 dehydration • 3.2 alkene • 3.3 concentrated • 3.4 haloalkane • 3.5 distillation • 3.6 reflux • 3.7 orange • 3.8 green
Q4.1 — Why concentrated acid?
Concentrated acid acts both as a catalyst and as a dehydrating agent. At high concentration, it drives equilibrium toward dehydration (water removal) rather than hydration (water addition). Dilute acid at lower temperature favours hydration, which is the reverse reaction.
Q4.2 — Why is water always produced in substitution with HX?
In the substitution reaction R–OH + HX → R–X + H2O, the –OH from the alcohol and the H from HX combine to form water. Since the halide X− displaces –OH, the expelled –OH must pick up a proton (from H–X) to become H2O. This follows directly from the law of conservation of mass — the oxygen must go somewhere.
Q4.3 — Why tertiary alcohols resist oxidation
Oxidation of an alcohol requires removing one H from the C–OH carbon and one H from the –OH group to form a C=O bond. In a tertiary alcohol, the C–OH carbon is bonded to three other carbon groups and carries no H atoms — there is no C–H bond available to break, so the oxidation cannot proceed.
Q4.4 — HX reactivity order
Order: HI > HBr > HCl (fastest to slowest). The C–I bond is weaker than C–Br or C–Cl, making formation of the C–I bond in the product (iodoalkane) more thermodynamically favourable. Iodide is also a better nucleophile than bromide or chloride.
Q5 — Sample concept map
Correct arrows from ALCOHOL include:
- ALCOHOL → ALKENE: dehydration • conc. H2SO4, ~170°C
- ALCOHOL → HALOALKANE: substitution with HX • reflux
- ALCOHOL (1°) → ALDEHYDE: oxidation with K2Cr2O7/H+ • distillation
- ALCOHOL (1°) → CARBOXYLIC ACID: oxidation with excess K2Cr2O7/H+ • reflux
- ALCOHOL (2°) → KETONE: oxidation with K2Cr2O7/H+ • reflux
Award 1 mark per correctly labelled arrow with both reaction name and condition (max 5).